Cavitation as an aid to rotary drilling



United States Patent 3,174,5l CAVITATION AS AN All) T0 RGTARY DRiLLiN-G Eugene L. Sterrett, 326 Elizabeth St., Findlay, @hio N0 Drawing. Filed Mar. 23, 1960, filer. No. 16,963 9 Claims. (6!. 175-65) This invention relates to rotary drilling of wells. The rotary drilling of oil, gas and waterwells consists of sinking a shaft or hole into the earth. The rock mass through which the well is drilled has heretofore been abraded and chipped away only by the downward pressure and cutting and grinding action of a revolving steel bit which may assume various forms. The cutting bit is revolved by a substantial steel pipe or drill stem extending to the surface where the torque is applied with adequate machinery common to the rotary drilling industry. The drill stem is hollow so that water, mud or many combinations of fluids can be circulated with pumps down through the drill stem to the drill bit and out into the Well through holes in the bit. This fluid sweeps under the bit, picks up loosened rocky material and carries it to the surface through the annular space between the drill stem and the walls of the well. These rocky cuttings settle out of the circulating drilling fluid at the surface when the flow rate is reduced to a sluggish rate.

The drilling fluid is thus circulated over and over. The aforegoing is a brief description of the rotary drilling technique as practiced currently of course there are many details known to the industry which influence the penetration rate.

The subject invention pertains to the addition of a capsule as hereinafter described to the circulating fluid just prior to the drilling fluids being pumped down the drill stem. This device reduces the grinding and chipping required of the bit to break up the rock at the bottom of the hole. In effect it actually does chipping, grinding and fracturing where it is most effective thereby supplementing the action of the bit. This increases the penetration rate with decreased wear on the drilling bit. This constitutes quite an economic advantage mostly due to the reduced time required to drill to any certain depth.

For many years, a form of erosion was noticed on turbine blades and pump impellers regardless of the type of metal. This erosion was quite severe and in some cases would completely destroy an impeller or turbine wheel in a few hours. At first, it was thought to be a form of corrosion associated with the liberation of oxygen but since then it has been proven to be a phenomenon known as cavitation. In hydraulic systems, cavitation occurs whenever the pressure at a point falls to or below the vapor-pressure of the liquid, whereupon masses of the vapor are formed and move along with the stream. If a section is then reached where the pressure increases, these vapor masses collapse and by so doing give rise to pressures of great intensity.

This same phenomenon can be caused to occur by rupturing a capsule which is an envelope containing low pressure gas or essentially a vacuum in an environment of high pressure liquid. The liquid rushing to fill the void so created also generates pressures of great intensity. In fact it is a way of creating cavitation artificially.

The capsule shell can be any material which has the tensile strength to withstand the external hydraulic pressure at essentially zero internal pressure, but it should also be of a material which is brittle or not ductile so when it does rupture its complete destruction is instantaneous. Cavitation is dependent upon the creation of an instantaneous voidage in a liquid under pressure therefore a gradual crushing of the capsule would not suffice. A material such as glass is ideal however any material which has similar physical characteristics such as the 3,174,561 Patented Mar. 23, 1965 plastic known by the trade name Bakelite, can be used to cause the cavitation phenomenon artificially. The rupture or failure of the capsule is caused by finally exceeding the tensile strength of capsule shell material with hydraulic external pressure, mechanical force or a combination of both.

One method to utilize this invention but not necessarily the only procedure as applied as an aid to rotary drilling is practiced by lubricating or injecting these cavitation capsules into the circulating drilling fluid stream just after it leaves the drilling fluid pump discharge and before it enters the drill stern as it is pumped to the drilling bit at the bottom of the well through the drill stem. Cavitation capsules can not be injected into the pump suction because they would be crushed by the mechanical action of the pump. They can be lubricated into the drilling fluid stream downstream of the pump. The term lubricated is common to the industry and means dropped into a second container by the force of gravity or buoyance from the first container which is maintained at equal pressure. These capsules are carried by the drilling fluid to the bottom of the drill stem and through the openings in the drill bit to the very bottom of the well. At the bottom of the well, these capsules are crushed by the combined forces of the mechanical action of the drill bit and the hydraulic pressure whereupon the extreme shock pressures of cavitation are created. These shock pressures of cavitation cause erosion, crumbling and fracturing of the rock formation just beneath the drill bit thereby aiding the cutting, crushing, and grinding action of the drill bit. The crumbling and erosion caused by the cavitation reduces the cutting and grinding requirements of the drill bit which allows it to penetrate the rock formation at a faster rate. Also, it reduces the wear on the drill bit since it does reduce the cutting requirements of the bit and allows the bit to knock free the then loosened rocky material thus, saving drill bits and time required to haul the drill stem out of the well to change the drill bit.

The size and shape of the capsule is dependent upon several things. The shape of the capsule should be essentially spherical in order to prevent its becoming wedged in restricted cross sectional areas of the drill stern and openings in the drill bit. The spherical shape also provides maximum strength against crushing by hy draulic pressure for a given shell thickness. The thinner the shell, the more voidage volume is available, per given outside diameter and in turn the greater the cavitation effect. The size of the capsule can vary from approximately one-quarter inch diameter to as large as one and onequarter inches. The maximum diameter is limited by the size of the internal cross section of drill stern and the size of the opening in the drill bit. The optimum size capsule used will depend on many variables such as type of drill bit, type of rock being drilled, size of hole being drilled, depth of hole, type of drilling fluid, rotation speed of the bit, circulation rate of the drilling fluid and weight applied to the drill bit. It is possible to crumble the rock at the bottom of the hole faster than the circulating fluid can carry it out to the surface. The many factors influencing the penetration rate must be coordinated properly for the best results. Normally, five-eighths inch diameter capsules are about right for most drilling conditions. The variation in cavitation severity can be controlled by varying the number of capsules lubricated or injected into the drilling fluid per minute. Perhaps a better way of expressing the capsule injection rate is according to bit rotation. In most cases, the bit is rotated at approximately sixty revolutions per minute. One capsule for each 360 degrees rotation of the bit is generally about the optimum injection rate. For harder more dense 3 rock, the injection rate can be increased to a capsule for each 90 degree rotation of the bit or larger capsules may be used.

As the hole is drilled deeper a given capsule becomes more effective because of the increased hydraulic pressure at the greater depth created by the hydrostatic head of the drilling fluid. Of course, the minimum shell thickness of the capsule must be greater for deeper drilling to resist the greater hydrostatic pressure. Generally, the capsule is crushed by the mechanical action of the bit rather than hydraulic pressure so the same capsule shell thickness is specified for a large range of depth or hydraulic pressure. The capsule shell thickness is also dependent upon the physical characteristics of the material from which the capsule is made.

It will be apparent that various other changes and modifications can be made from the specific details discussed herein without departing from the spirit and scope of the attached claims.

What I claim is:

1. A method for improving the rotary drilling penetration rate in a well drilling system wherein a drill bit is rotated by a hollow stem to drill the well and circulating drilling fluid is circulated downwardly through the stem and the bit and outwardly between the stem and the well, which method comprises adding to the circulating drilling fluid a plurality of hollow and frangible capsules which are of a size and shape as to be freely carried by the drilling fluid through the stem and drill and create cavitation when the capsules are broken by the drill bit at the bottom of the well.

2. A method for improving the rotary drilling penetration rate in a well drilling system wherein a drill bit is rotated by a hollow stem to drill the well and circulating drilling fluid is circulated downwardly through the stem and the bit and outwardly between the stem and the well, which method comprises adding to the circulating drilling fluid hollow glass spheres which are of a size and shape as to be freely carried by the drilling fluid through the stem and drill and which create extreme pressure waves when they are ruptured by the drill bit at the bottom of the well.

3. A method for improving the rotary drilling penetration rate in a well drilling system wherein a drill bit is rotated by a hollow stem to drill the well and circulating drilling fluid is circulated downwardly through the stem and the bit and outwardly between the stem and the well, which method comprises continually adding to the circulating drilling fluid hollow and frangible air filled spheres which are of a size and shape as to be freely carried by the drilling fluid through the stem and drill and which fracture the rock beneath the drilling bit when they are ruptured.

4. A method for improving the rotary drilling penetration rate in a well drilling system wherein a drill bit is rotated by a hollow stem to drill the Well and circulating drilling fluid is circulated downwardly through the stern and the bit and outwardly between the stem and the well,

which method comprises continually adding to the circulating drilling fluid hollow evacuated glass spheres which are of a size and shape as to be freely carried by the drilling fluid through the stern and drill and which fracture the rock formation at the bottom of the well by cavitation when they are broken as they are swept under the rotating drill bit.

5. A method for improving the rotary drilling penetration rate in a well drilling system wherein a drill bit is rotated by a hollow stem to drill the well and circulating drilling fluid is circulated downwardly through the stem and the bit and outwardly between the stem and the well, which method comprises continually adding to the circulating drilling fluid hollow glass capsules which implode when crushed by the drill bit and which are of a size and shape as to be freely carried by the drilling fluid through the stem and drill and which grind and fracture the rock formation beneath the drill bit when they are broken and swept under the drill bit.

6. In the drilling of wells by rotating a drill bit and continuously circulating drilling fluid through a hollow stem to the drill bit, the method which comprises adding a plurality of destructible capsules having gaseous voids therein to the circulating drilling fluid, said capsules being of a size and shape as to be freely carried by the drilling fluid through the stem and drill, said capsules being made of a material and having a thickness such that they are broken -y the combined hydraulic pressure of the head of circulating fluid and the mechanical force at the bit.

7. The combination set forth in claim 6 wherein said capsules are spherical.

8. The combination set forth in claim 7 wherein said capsules have a diameter ranging between approximately one-quarter inch and one and one-quarter inches.

9. The combination set forth in claim 7 wherein said capsules have a diameter of approximately inch.

References tilted by the Examiner UNITED STATES PATENTS OTHER REFERENCES Setser, Donald: Implosion Technique Improves Fracturing Performance, in World Oil, March 1960, pp.

CHARLES E. OCONNELL, Primary Examiner. 

1. A METHOD FOR IMPROVING THE ROTARY DRILLING PENETRATION RATE IN A WELL DRILLING SYSTEM WHEREIN A DRILL BIT IS ROTATED BY A HOLLOW STEM TO DRILL THE WELL AND CIRCULATING DRILLING FLUID IS CIRCULATED DOWNWARDLY THROUGH THE STEM AND THE BIT AND OUTWARDLY BETWEEN THE STEM AND THE WELL, WHICH METHOD COMPRISES ADDING TO THE FRAGIBLE CAPSULES ING FLUID A PLURALITY OF HOLLOW AND FRANGIBLE CAPSULES WHICH ARE OF A SIZE AND SHAPED AS TO BE FREELY CARRIED BY THE DRILLING FLUID THROUGH THE STEM AND DRILL AND CREATE CAVITATION WHEN THE CAPSULES ARE BROKEN BY THE DRILL BIT AT THE BOTTOM OF THE WELL. 